1. Field of the Invention
The present invention relates to a vertical (vertical-electrode-type) GaN-based light emitting diode (hereinafter, referred to as ‘LED’), which can minimize a loss of photons generated in an active layer so as to increase the light-extraction efficiency, thereby increasing external quantum efficiency.
2. Description of the Related Art
Generally, a GaN-based LED is grown on a sapphire substrate, but the sapphire substrate is a rigid nonconductor and has poor thermal conductivity. Therefore, there is a limitation in reducing the manufacturing costs by decreasing the size of a GaN-based LED, or improving the optical power and chip characteristic. Particularly, because the application of a high current is essential for achieving high power LED, it is important to solve a heat-sink problem of the LED. To solve this problem, there has been proposed a vertical GaN-based LED in which a sapphire substrate is removed using a laser lift-off (LLO).
However, the conventional vertical GaN-based LED has a problem in that photons generated from an active layer are emitted to the outside of the LED. That is, the external quantum efficiency is degraded.
Now, the problem of the conventional vertical GaN-based LED will be described in detail with reference to FIGS. 1 to 3.
FIG. 1 is a perspective view illustrating the structure of the conventional vertical GaN-based LED. FIG. 2 is a plan view illustrating the structure of the vertical GaN-based LED of FIG. 1. FIG. 3 is a sectional view illustrating the structure of the vertical GaN-based LED, taken along III-III′ line of FIG. 2.
Referring to FIGS. 1 to 3, the conventional vertical GaN-based LED includes an n-type bonding pad 110, an negative (n-) electrode 120 formed under the n-type bonding pad 110, an n-type GaN layer 130 formed under the n-electrode 120, an active layer 140 formed under the n-type GaN layer 130, a p-type GaN layer 150 formed under the active layer 150, and a positive (p-) electrode 160 formed under the p-type GaN layer 150, and a support layer 170 formed under the p-electrode 160.
The n-electrode 120 and p-electrode 160 not only serve as an electrode, but also serve to reflect light. Therefore, the n-electrode 120 and p-electrode 160 are preferably formed of metal with high reflectance.
In the conventional vertical GaN-based LED, photons generated in the active layer 140 are directly emitted to a light emitting surface or are totally reflected into the light emitting surface by the p-electrode 160 formed under the active layer 140 so as to be emitted, as shown in FIG. 3.
However, when the photons are totally reflected into the light emitting surface through the p-electrode 160, a loss of light generated in the active layer can occur. More specifically, the conventional vertical GaN-based LED has a path where light generated in the active layer is emitted into the light emitting surface, and a horizontal path is longer than a vertical path. Therefore, photons emitted to the side of the LED are absorbed or scattered while being totally reflected between the p-electrode and the light emitting surface.
In other words, since horizontal paths of the active layer, the p-type GaN layer, and the n-type GaN layer are long in the conventional vertical GaN-based LED, a critical angle α at which light is laterally emitted is so small that a loss of photons is inevitably caused by the total reflection between the p-electrode and the light emitting surface. Therefore, the external quantum efficiency decreases.